Convert Kelvin into Celsius in Swift - ios

Question

I'm trying to retrieve the temperature from the users current location.
I am using the API from OpenWeatherMap. The problem is, they provide the temperature in Kelvin as default, and I would like it in Celsius.
I understand that I just need to subtract 273.15 from the kelvin value....? But i'm struggling to figure out where to do that.
My code for setting my labels:
var jsonData: AnyObject?
func setLabels(weatherData: NSData) {
do {
self.jsonData = try NSJSONSerialization.JSONObjectWithData(weatherData, options: []) as! NSDictionary
} catch {
//handle error here
}
if let name = jsonData!["name"] as? String {
locationLabel.text = "using your current location, \(name)"
}
if let main = jsonData!["main"] as? NSDictionary {
if let temperature = main["temp"] as? Double {
self.tempLabel.text = String(format: "%.0f", temperature)
}
}
}
Can anyone help me get this right please, as I'm really not sure where to start, thanks.
Let me know if you need to see more of my code.

Answer 1

if let kelvinTemp = main["temp"] as? Double {
let celsiusTemp = kelvinTemp - 273.15
self.tempLabel.text = String(format: "%.0f", celsiusTemp)
}
or simply
self.tempLabel.text = String(format: "%.0f", temperature - 273.15)

Answer 2

For Swift 4.2:
Use a Measurement Formatter.
let mf = MeasurementFormatter()
This method converts one temperature type (Kelvin, Celsius, Fahrenheit) to another:
func convertTemp(temp: Double, from inputTempType: UnitTemperature, to outputTempType: UnitTemperature) -> String {
mf.numberFormatter.maximumFractionDigits = 0
mf.unitOptions = .providedUnit
let input = Measurement(value: temp, unit: inputTempType)
let output = input.converted(to: outputTempType)
return mf.string(from: output)
}
Usage:
let temperature = 291.0
let celsius = convertTemp(temp: temperature, from: .kelvin, to: .celsius) // 18°C
let fahrenheit = convertTemp(temp: temperature, from: .kelvin, to: .fahrenheit) // 64°F
To output the localized temperature format, remove the line mf.unitOptions = .providedUnit

Answer 3

From the code above, it seems to me the right place to do this would be right after you get the temperature
if let temperatureInKelvin = main["temp"] as? Double {
let temperatureInCelsius = temperatureInKelvin - 273.15
self.tempLabel.text = String(format: "%.0f", temperature)
}
In the future though, I would probably parse your JSON values in a separate class and store them in a model object which you can call later on.

Answer 4

A simpler example of the above handy function (updated for Swift 5.3) would be something like:
func convertTemperature(temp: Double, from inputTempType: UnitTemperature, to outputTempType: UnitTemperature) -> Double {
let input = Measurement(value: temp, unit: inputTempType)
let output = input.converted(to: outputTempType)
return output.value
}

Answer 5

Here:
self.tempLabel.text = String(format: "%.0f", temperature - 273.15)
or you can do it here (pseudo syntax as I don't know Swift that well):
if let temperature = (main["temp"] as? Double) - 273.15 {
self.tempLabel.text = String(format: "%.0f", temperature)
}